Conventionally, electrical sensors have been generally known as a sensor for measuring physical quantities of an object such as movement and temperature of the object, and pressure applying to or applied by the object. The electrical sensor transmits, to a processing section, an electrical signal generated in a measuring section (sensor head) in accordance with a physical quantity. Then, the transmitted electrical signal is converted into the physical quantity in the processing section. When a distance between the measuring section and the processing section is large, the electrical signal is susceptible to an electromagnetic noise, whereby measurement accuracy of the electrical sensor is deteriorated. Further, the electrical sensor requires supplying power to the measuring section located in a distance, therefore is unsuitable for measuring, for example, a water level (water pressure) of a dam, a water level (water pressure) of a subsoil drainage pipe, or a water level (pressure) of combustible liquefied natural gas in a storage tank, for example.
In contrast, in an optical sensor including an optical fiber, a measuring section is supplied with light through the optical fiber. Instead of the electrical signal, an optical signal in accordance with a physical quantity is transmitted to a processing section from the measuring section through the optical fiber. Therefore, the optical sensor is capable of transmitting a signal from the measuring section without influence of the electromagnetic noise, and the measurement can be performed with high accuracy (for example, see Patent Literatures 1 to 10).
FIG. 10 is a schematic view illustrating a configuration of a conventional optical pressure sensor as described in Patent Literature 10 and the like. An optical pressure sensor 101 includes a sensor head 102, an optical device 103, and a calculation section 104. The optical device 103 includes a light source 105, a first photodetector 106, a second photodetector 107, a first amplifier 108, and a second amplifier 109.
The sensor head 102 includes a light-emitting optical fiber 110 for transmitting light emitted from the light source 105 to the sensor head 102, and a light-receiving first optical fiber 111 and a light-receiving second optical fiber 112 that transmit the light to the optical device 103 from the sensor head 102. The light-emitting optical fiber 110, the light-receiving first optical fiber 111, and the light-receiving second optical fiber 112 are held by an optical-fiber holding section 113 in the sensor head 102. The sensor head 102 includes a diaphragm 114 that is formed from a thin metal plate and located at a position facing to an end surface of the light-emitting optical fiber 110. The diaphragm 114 includes a reflecting plate 115 located on a surface facing to the end surface of the light-emitting optical fiber 110. The diaphragm 114 is deformed by a pressure applying to an outside thereof, and the position of the reflecting plate 115 is accordingly moved.
In the example illustrated in FIG. 10, the sensor head 102 is provided under water, and the optical pressure sensor 101 measures a pressure (water pressure) applying to the diaphragm 114 so that information of a water level can be obtained. The light emitted from the light source 105 is transmitted to the sensor head 102 through the light-emitting optical fiber 110, then the transmitted light is emitted from the end surface of the light-emitting optical fiber 110. Thereafter, the light reflects on a reflecting surface of the reflecting plate 115, and enters end surfaces of the light-receiving first optical fiber 111 and the light-receiving second optical fiber 112. The light entered the end surface of the light-receiving first optical fiber 111 is transmitted to the first photodetector 106 through the light-receiving first optical fiber 111, and the light entered the end surface of the light-receiving second optical fiber 112 is transmitted to the second photodetector 107 through the light-receiving second optical fiber 112.
FIG. 11 is an enlarged sectional view of a main part of the sensor head. The diaphragm is not shown in FIG. 11. The light-receiving first optical fiber 111 and the light-receiving second optical fiber 112 are provided so as to be parallel to each other. The light-emitting optical fiber 110 and the light-receiving first optical fiber 111, and the light-emitting optical fiber 110 and the light-receiving second optical fiber 112 are each tilted so as to be symmetric with respect to a normal line of the reflecting surface of the reflecting plate 115 (specifically, so that each of angles θ with respect to the normal lines of the reflecting surfaces is 5°). The end surfaces of the light-emitting optical fiber 110, the light-receiving first optical fiber 111, and the light-receiving second optical fiber 112 face to the reflecting surface of the reflecting plate 115.
The light emitted from the end surface of the light-emitting optical fiber 110 progresses while expanding radially with an optical axis in the center, then reflects on the reflecting surface of the reflecting plate 115, to thereby enter into the light-receiving first optical fiber 111 and the light-receiving second optical fiber 112 through the respective end surfaces thereof. Meanwhile, the position of the optical axis of the light thus reflected changes in accordance with the position of the reflecting plate 115. Because of this, an intensity of the light entering the light-receiving first optical fiber 111 and an intensity of the light entering the light-receiving second optical fiber 112 change in accordance with the position of the reflecting plate 115.
The first photodetector 106 (see FIG. 10) receives the light thus transmitted, and generate an electrical signal corresponding to the light thus received. Meanwhile, the second photodetector 107 receives the light thus transmitted, and generate an electrical signal corresponding to the light thus received. The electrical signals generated in the first photodetector 106 and the second photodetector 107 are supplied to the first amplifier 108 and the second amplifier 109, respectively.
The first amplifier 108 and the second amplifier 109 each amplify the electrical signals that they respectively received. Then, the first amplifier 108 and the second amplifier 109 each send, to the calculation section 104, the electrical signals thus amplified.
Putting that the intensity of the light received by the first photodetector 106 (in other words, the light intensity indicated by the electrical signal supplied by the first amplifier 108) is P1, the intensity of the light received by the second photodetector 107 (in other words, light intensity indicated by the electrical signal supplied by the second amplifier 109) is P2, and a light intensity ratio calculated in the calculation section 104 based on the light intensities P1 and P2 is F value, the F value is obtained by calculating the following equation: F(P1, P2)=(P1−P2)/(P1+P2). The F value thus obtained is compared with, for example, a table calibrated in advance. In this way, a pressure and a depth of water at the position of the diaphragm 114 can be obtained.